CA1306819C - Aqueous electrocoating baths for cathodic electrocoating and preparation thereof - Google Patents

Abstract

Abstract:
Aqueous electrocoating baths for cathodic electrocoating and preparation thereof The invention relates to aqueous electrocoating baths for cathodic electrocoating which contain, as cathodically depositable binders, modified epoxide-amine adducts and have been prepared by reacting (1) (A) Polyepoxides and (B) Compounds which contain one or more, preferably 2, hydroxyl groups bonded to aromatic and/or (cyclo)aliphatic molecule fragments per molecule, in the presence of basic amine catalysts to form epoxy-containing intermediates and (2) subsequently reacting these intermediates optionally with further customary modifiers and finally with (C) primary and/or secondary (poly)amines, neutraliz-ing the reaction products thus obtained with acids, dispersing the products in an aqueous medium and further processing this dispersion into electrocoating baths using well-known methods. The electrocoating baths according to the invention are distinguished by the fact that the reaction products formed from (A), (B) and (C) were dispersed in an aqueous medium before the neutralization with acid and subsequently the dispersion thus obtained had added to it at least one anion exchanger which is reactive toward chloride ions and which was separated off on conclusion of the exchange reaction.

Description

1~06~9 PAT 86 071 ~ay 20, 1986 BASF Lacke+Farben AG, MUnster ~queous electrocoating baths for cathodic electrocoating _ and preparation thereof The present invention relates to aqueous electro-coating baths for cathodic electrocoating which contain, as cathodically depositable binders, modified epoxide-amine adducts and have been prepared by reacting (1) (A) polyepoxides and (B) compounds which contain one or more, preferably 2, hydroxyl groups bonded to aromatic and/or (cyclo)aliphatic molecule fragments per molecule in the presence of basic amine catalysts to form epoxy-containing intermediates and (2) subsequently reacting these intermediates optionally with further customary modifiers and finally with (C) primary and/or secondary (poly)amines, neutraliz-ing the reaction products thus obtained with acids, d;spersing the products in an aqueous medium and further processing th;s dispersion into electro-coating baths using ~ell-known methods.
Cationic electrocoating is used frequently in particular for basecoating, whereby water-dilutable syn-thetic resins having cationic groups are applied to electroconductive bodies with the aid of direct current.
Electrocoating baths of the type described above are disclosed for example in the following patent docu-ments: US Patents 3,799,854, 3,984,299, 4,031,050, 4,252,703, k,332,711 and German Patent 3,108,073.
It is known that qualitatively excellent coatings can be obtained with coating systems of this kind.
Unfortunately, the electrocoating baths in ques-tion also contain undesirable chloride ions, which, on exceeding a concentration limit dependent on several parameters (eg. pH of the electrocoating bath), cause corrosion in the electrocoating equipment, in particular ~;306i3~9 on the anodes (eg. pitting of stainless steel electrodes).
The troublesome chloride ions come in the main from the polyepoxides ~hich are used as component (A) in the synthesis of the binders and which are contaminated S with chlorine-containing by-products.
These by-products can contain for example the following structural elements:

C;

O-CH-CH7-Cl CH,OH

{~} O CH2- ~CH-CH20H

~ O- CH2-~ H-CH2 -Cl o \ /

~3~6~319 ~ 3 - 27293-8 The chlorine content of commercially available polyepoxides is, on account of the presence of such by-products, between 0.15 and 0.5~.
In German AusLegeschrift 2,751,498 it is stated that the addition of nitrate and/or nitrite ions to the electrocoating bath reduces the corrosion damage caused by the presence of chLoride ions.
Neither this process nor purification of the polyepoxides contaminated with chlorine-containing by-products is carried out in practice, owing to economicand/or technical difficulties.
It is an object of the present invention to pro-vide electrocoating baths of the type described at the beginning, which have lower chloride ion concentrations than the prior art baths.
~ e have found, surprisingly, that this object can be achieved by preparing electrocodting baths of the type described abover wherein the reaction products formed from (A), (a) and ~C) have been dispersed in an aqueous medium before the neutralization with acid and subsequently the dispersion thus obtained has added to it at least one anion exchanger which is reactive toward chloride ions and which has been separated off on conclusion of the exchange reaction.
The present invention also relates to a process ~or preparing aqueous electrocoating baths for cathodic elec-trocoating by reacting (1) (A) polyepoxides and (B) compounds which contain one or more, preferably 3û 2, hydroxyl groups bonded to aromatic and/or (cyclo)aliphatic molecule fragments per molecule in the presence of basic amine catalysts to give epoxy-containing intermediates and .
(2) subsequently reacting these intermediates optionally with further customary modifiers and finally with (C) primary and/or secondary (poly)amines, neutraliz-ing the reaction products thus obtained with acids, 13~6~319 4 27~93-8 di~persing the prod~cts in an a~ueous medium and further processing these dispersions using well-know me~hods into electrocoating baths, which comprises dispersing the reaction products formed from (A), (B) and (C) in an aqueous medium before the neutralization with acid and subsequently adding to the dispersion thus obtained at least one anion exchanger which is reactive toward chloride ions and which is separated off on conclusion of the exchange reaction.
Thus, according to one aspect, the invention provides an aqueous electrocoating bath for cathodic electrocoating which contains, as cathodically depositable binder, an epoxideamlne adduct prepared by reacting 1 (A) a polyepoxide and (B) a compound which contains one or more hydroxyl groups bonded to an aromatic, aliphatic or cycloaliphatic moiety, in the presence of a basic amine catalyst to form an epoxy-containing reactant and (2) subsequently reacting the epoxy-containing reactant with (C) a primary or secondary amine, neutralizing the reaction product thus obtained with acid, dispersing the products in an aqueous medium and incorporating this dispersion into an electrocoating bath, wherein the reaction product formed from (A), (B) and (C) have been dispersed in an aqueous medium before the neutralization with acid and the dispersion thus obtained has had added to it at least one anion exchanger which is reactive toward chloride ions and which has been separated off on conclusion of the exchange reaction.
According to another aspect, the invention provides a process for preparing an aqueous electrocoating bath for cathodic electrocoating, which process comprises reacting an epoxy-~306819 4a 27293-8 containing reactant obtained by reacting a polyepoxide with the hydroxyl compound which contains one or more hydroxyl groups bonded to an aromatic, aliphatic or cycloaliphatic moiety in the presence of a basic amine catalyst with a primary or secondary amine, dispersing the product of reaction in a aqueous medium, contacting the aqueous medium with an ion exchanger which is reactive to chloride ions to remove chloride ions, acidifying the aqueous medium to neutralize it and incorporating the neutralized aqueous medium in an aqueous electrocoating bath.
To prepare the electrocoating baths according to the invention, the cathodically depositable binder is synthesized by well-known methods in a reaction medium which predominantly consists of organic solvents.
In this preparation, first of all, epoxy-containing intermediates are prepared from components (A) and (B) under basic amine catalysis.
Component (A) can be any compound whose molecules contain on average more than 1 epoxy group. Preference is given to those compounds which contain 2 epoxy groups per molecule and have a relatively low molecular weight of at most 750, preferably 350-500.
Particularly preferred epoxy compounds are polyglycidyl ethers of polyphenols prepared from polyphenols and epihalohydrins. The polyphenol is preferably bisphenol A.
It is also possible to use polyglycidyl esters of polycarboxylic acids. Typical examples are glycidyl adipate and glycidyl phthalate.

~;~06~319 4b 27293-8 It is also to use hydantoin expoxidized polybutadiene and polyepoxy compounds which are obtained by epoxidizing an olefinically unsaturated alicyclic compound.
Component tB) is a compound which contains one or more, preferably 2, hydroxyl groups bonded to aromatic and~or (cyclo)aliphatic molecule fragments per molecule.
Suitable compounds for use as component ~) include not only low molecular weight but also high molecular weight compounds.

1306~3~9 Suitable low molecular weight (E) components consist of phenolic, aliphatic and/or alicyclic poly-functional alcohols having a molecular weight below 350.
Specific examples are: diols, such as ethylene glycol, dipropylene glycol, triglycol, 1,2-propanediol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 2-methyl-2-propyl-1,3-propanediol, 1,2-butanediol, 1,4-butanediol, 2-ethyl-1,4-butanediol, but-2-ene-1,4 diol,1,2-pentanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 2-hydroxyethyl hydroxyacetate, 2,2-dimethyl-3-hydroxy-propyl-2,2-dimethyl hydroxypropionate, 4,4'-methylenebis-cyclohexanol and 4,4'-isopropylidenebiscyclohexanol.
Some preferred diols are 2,2-dimethyl-1,3-propanediol and 3-methyl-1,5-pentanediol.
Examples of relatively high mo~ecular weight (B~
components are polyester polyols, polyether polyols or polycaprolactone Polyols of varying functionalities and molecular weights.
Polyalkylene ether polyols suitable for use as component ~8) conform to the general- formula:-H r O --t CHR )n J-- OH

in which R is hydrogen or a low alkyl radical, if desiredwith various substituents, n is 2-6 and m is 3-50 or even higher. Examples are polytoxytetramethylene) glycols and poly(oxyethylene) glycols.
The preferred polyalkylene ether polyols are poly-~oxytetramethylene) glycols having a molecular weight within the range of 350-1,000.
Polyester polyols can likewise be used as t8) components. The polyester polyols can be prepared by polyesterification of organic polycarboxylic acids or their anhydrides with organic polyols which contain pri-mary hydroxyl groups. Customarily the polycarboxylic acids and the polyols are aliphatic or aromatic dicar-boxylic acids and diols.

~ ~19 The diols used for preparing the polyesters in-clude alkylene glycols such as ethylene glycol, butylene gLycol, neopentylglycol and other glycols such as cyclo-hexanedimethanol.
The acid component of the polyester cons;sts pri-marily of Low molecular weight carboxylic acids or anhyd-rides theresf with 2-18 carbon atoms in the molecule.
Suitable acids are for example phthalic acid, isoPhthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydro-1û phthalic acid, adipic acid, azelaic acid, sebacic acid and glutaric acid. In place of these acids it is also pos-sible to use ~he anhydrides, insofar as they exist.
It is further possible to use as component (3) polyester polyols which are derived from lactones. These materials are obtained by reacting an ~-caprolactone with a polyol. Products of this type are described in US
Patent 3,169,945.
The polylactone polyols which are obtained by this reaction are distinguished by the presence of a terminal hydroxyl group and by recurring polyester por-tions which are derived from the lactone. These recurring molecule fragments can conform to the formula _ C - (CHR)n 2 in which n is at least 4, preferably 4-6, and the sub-stituent is hydrogen, an alkyl radical, a cycloalkyl radical or an alkoxy radical.
Suitable for use as basic amine catalysts are in principle all compounds containing one or more basic nitrogen atoms.
Preference is given to using tertiary amines, such as, for example, N,N-dimethylbenzylamine, tributyl-amine, dimethylcyclohexylamine and dimethyl-C12/C14-amine (C12/C14 signifies an aliphatic chain containing 12-14 carbon atoms).

_ 7 ~306~319 The basic amine catalyst is in generaL used in an amount of 0.1-2~ by weight, based on the intermediate formed from components (A) and (B).
The reaction between components (A) and (B) is carried out at temperatures between 1ûû and 190C, pre-ferably between 115 and 185C.
The epoxy-containing intermediates formed from components (A) and (~) are reacted with further customary modifiers requiring no basic amine catalysis and there-after with component (C).
Component (C) can be a primary and/or secondaryamine, secondary amines being particularly preferred com-ponents tC).
Preferably the amine should be a water-solubLe compound. Examples of such amines are mono- and dialkyl-amines, such as methylamine, ethylamine, propylamine, butylamine, dimethylamine, diethylamine, dipropylamine, methylbutylamine and the like. Also suitable are alkanol-amines, for example methylethanolamine, diethanolamine, and the like. Also suitable are dialkylaminoalkylamines, for example dimethylaminoethylamine, diethylaminopropyl-amine, dimethylaminopropylamine and the like. In most cases use is made of relatively low molecular weight amines, but it is also possible to use higher molecular weight monoamines.
Polyamines having primary and secondary amino groups can be reacted with the epoxy groups in the form of ket;m;nes. The ketimines are prepared from the poly-amines in a conventional manner.
The amines can also contain other groups, but these groups should not interf~re with the reaction of the amine wi~h the epoxy group, nor should they lead to a gelling of the reaction mixture.
The reaction between am;nes and epoxy-containing compounds frequently sets in just on mixing the reactants.
Depending on the desired course of the reaction it is advis-able, in particular to complete conversion, to raise the ~306~319 reaction temperature to 50-150C.
Following reaction of the intermediates formed from components (A) and (8) with component (C), the resin solution obtained is dispersed in the absence of acid in an aqueous medium.
The result obtained is an unstable dispersion which is surprisingly finely divided, readily stirrable and also readily filterable. This dispersion is if de-sired cooled down to 40 to 50C and subsequently has added to it at least one anion exchanger reactive to chloride ions.
The dispersion in question is advantageously pre-pared using a portion of the water required for preparing the complete coating bath and is stirred not only before t~e addition of the anion exchanger but also during the exchange process.
In the process according to the invention, it is possible in princ;ple to use any known anion exchanger which is inert toward the binder intermediate present in the dispersion to be treated.
The separation of binder intermediate dispersion and anion exchanger can be carried out by any desired process; preferably, however, separation is effected with the aid of a technically simple filtration.
It was surpr;s;ng and not foreseeable that the process accord;ng to the invention produces dispersions from which the anion exchanger is readily separable with-out significant technical difficulties.
The anion exchangers usable in the process accord-ing to the invention can be anion exchangers based on synthetic resin which contain, as carriers of the basic properties, primary, secondary or tertiary amino groups, quaternary ammonium groups or nitrogen integrated in a ring structure.
Examples which may be mentioned are crosslinked polycondensation products of amines or phenoles and formaldehyde as well as crosslinked styrene polymers ~306~3~9 containing basic groups.
Preference is given to using crosslinked styrene polymers containing basic groups and present in bead form.
The anion exchanger is added in such an amount -that the total exchange capacity is S to 20 times higherthan the amount of chloride ions contained in the dis-persion.
The exchange process is in general complete after 10 to 60 minutes. Thereafter the ion exchanger is sePara-ted from the purified dispersion, preferably by filtra-tion, and, after regeneration, is available for another purification cycle.
After the ion exchanger has been separated off, the dispersion is stabilized by adding water-soluble acids (for example formic acid, lactic acid, propionic acid) and further processed by well-known methods into an a-queous electrocoating bath.
The electrocoating baths according to the invention can contain customary additives, for example crosslinking agents, coalescent solvents, pigments, surface-active agents, crosslinking catalysts, antioxidants, fillers, antifoams etc.
By means of the process according to the inven-tion, which is technically s;mple to carry out, ;t is possible to obtain electrocoating baths which have a chloride ion content wh;ch is significantly below that which is found ;n comparable electrocoating baths in the preparation of which the measure according to the inven-tion has not been carried out.
The invention is explained in more detail in the following examples. Parts and percentages are by weight, unless expressly stated otherwise.
Example 1 Preparation of a binder as described in 35 EP 70,550, Example B

~06~319 Weight (g) - Epikote 8291) 727.6 - Capa~2oo2) 268.4 - xylene 36.1 S - bisphenol A 197.8 - dimethylbenzylamine 3.8 - isocyanate crosslinking agent3)901.3 - diketimine from MIBK and diethylenetriamine, 75~ strength in MIBK 73.4 10 - N-methylethanolamine 59.1 - hexylglycol 76.5 - acetic acid 33 5 - emulsifier mixture4) 29.4 - deionized water 1,793.1 1) Epoxy resin based on bisphenol A from Shell Chemie, epoxy equivalent 188 2) PolycaproLactonediol from Interox Chemical 3) Isocyanate crosslinking agent based on toluylene diiso-cyanate with butylglycol capping and reaction with trimethylolpropane in a ratio of 3 : 1, dissolved in a mixture of MIBK and n-butanol (9 : 1) to 70% solids.

4) Emulsifier mixture based on Geigy Amin C ~ (Geigy Industrial Chemicals) 120 parts, Surfynol 104 ~ (Air Products and Chemicals) 120 parts, butylglycol 120 parts and 221 parts of deionized water with 19 parts of glacial acetic acid.

MIaK = methyl isobutyl ketone.
Method of preparation ,~ ~
Epikote~r829, Capa 200 and xylene are introduced into a reaction vessel and heated under N2 protective gas to 210C. Water is then separated off for half an hour.
The batch is then cooled down to 150C, and bisphenol A
and 1.6 parts of dimethylbenzylamine are added. This is followed by heating to 180C and maintaining that tem-perature for half an hour. The temperature is then re-duced to 130C, and the remainder of dimethylbenzylamine -~ rrQd~-~Rr¦~

is added. The temperature is then maintained for 2 1/2 hours, when the isocyanate crosslinking agent, diketimine and N-methylethanolamine are added, and the temperature is then ma;ntained at 110C for half an hour. The hexyl-glycol is then added. The reaction mixture is then dis-persed in the deionized water which contains the glacial acetic acid and emulsifier mixture. A vacuum is then applied to remove the very volatile organic solvents. A
solids content of 36% is set.
The chloride content of this dispersion is then determined argentometrically. It is found to be 63 ppm of chloride.
Example 2 The binder preparation of Example 1 is repeated, except that the reaction mixture is initially dispersed in only 950 9 of water and cooled to 45C. 45 ml of a commercially available anion exchanger in OH form (Dowex 1X2 ~, particle size SO to 100 ~m) are then added. The anion exchanger is filtered off after 30 minutes. The acetic acid and the emulsifier mixture are then added and mixed in for 15 minutes. The remaining 843 9 of water are then added. The argentometric chloride determination indicates a value of 29 ppm of chloride for this dispersion.
Example 3 Preparation of a binder in accordance with German Patent 3,108,û73, Example Z.
A react;on vessel ;s charged w;th 1,093 parts of Araldite~ Y 2600 (epoxy equivalent weight EEW = 188, epoxy resin based on b;sphenol A from Ciba Geigy), 151 parts of neopentylglycol and 4.9 parts of dimethylbenzyl-amine. The temperature is raised to 131C and maintained unt;l an EEW of 415 is reached. 398 parts of Capa~-200 (see Example 1) are then added, followed by a further 3.8 parts of dimethylbenzylamine. The temperature is mainta;ned at 131C until an EEW of 1,030 is reached. 1,274 parts of a crosslinking agent (see Example 1) and 112 parts of the diketimine likewise mentioned in Example 1 as well as 86 ;3~ Tr"cl~rk parts of N-methylethanolamine are then added, and the temperature is maintained at 112C for 1 hour. 135 parts of phenoxypropanol and 40 parts of methoxypropanol are then added and mixed in for 15 minutes. This resin solution is dispersed in 3,247 parts of water, 23.5 parts of glacial acetic acid and 21 parts of emulsifier mixture (see Example 1).
The low-boiling solvents are then removed in vacuo and a solids content of 35% is set.
The argentometrically determined chloride content of this dispersion is found to be 127 ppm of chloride.
Example 4 The binder preparation of Example 3 is repeated, except that the resin soluéion is initially dispersed in only 1,800 9 of water. After cooling to 40C, 60 ml of the anion exchanger described in Example 2 are mixed in.
The anion exchanger is filtered off after 45 minutes.
The dispersion is then worked up with a glacial acetic acid, emulsifying mixture and the remaining water as described.
The argentometric chloride determination indicates a value of 61 ppm of chloride for this dispersion.

Claims (8)

1. An aqueous electrocoating bath for cathodic electrocoat-ing which contains, as cathodically depositable binder, an epoxide-amine adduct prepared by reacting (1) (A) a polyepoxide and (B) a compound which contains one or more hydroxyl groups bonded to an aromatic, aliphatic or cycloaliphatic moiety, in the presence of a basic amine catalyst to form an epoxy-containing reactant and (2) subsequently reacting the epoxy-containing reactant with (C) a primary or secondary amine, neutralizing the reaction product thus obtained with acid, dispersing the products in an aqueous medium and incorporating this dispersion into an electrocoating bath, wherein the reaction product formed from (A), (B) and (C) have been dis-persed in an aqueous medium before the neutralization with acid and the dispersion thus obtained has had added to it at least one anion exchanger which is reactive toward chloride ions and which has been separated off on conclusion of the exchange reaction.

2. An aqueous electrocoating bath according to claim 1 wherein the hydroxyl-containing compound has two hydroxyl groups per aromatic, aliphatic or cycloaliphatic moiety.

3. An aqueous electrocoating bath according to claim 1 wherein the amount of anion exchanger added to the aqueous dis-persion was such that the total exchange capacity was 5 to 20 times higher than the amount of chloride ions present in the dis-persion.

4. A process for preparing an aqueous electrocoating bath for cathodic electrocoating, which process comprises reacting an epoxy-containing reactant obtained by reacting a polyepoxide with the hydroxyl compound which contains one or more hydroxyl groups bonded to an aromatic, aliphatic or cycloaliphatic moiety in the presence of a basic amine catalyst with a primary or secondary amine, dispersing the product of reaction in an aqueous medium, contacting the aqueous medium with an ion exchanger which is re-active to chloride ions to remove chloride ions, acidifying the aqueous medium to neutralize it and incorporating the neutralized aqueous medium in an aqueous electrocoating bath.

5. A process according to claim 4, wherein the hydroxyl compound contains two hydroxyl groups per aromatic, aliphatic or cycloaliphatic moiety.

6. A process according to claim 4 or 5, wherein the primary or secondary amine is added to the reaction mixture in the form of a ketimine.

7. A process according to claim 4 or 5, wherein the amount of an anion exchanger which contacts the aqueous dispersion has a total exchange capacity 5 to 20 times higher than the amount of chloride ions initially present in the dispersion.

8. A process according to claim 4 or 5 wherein the anionic exchanger is separated from the aqueous dispersion by filtration.